Operators

PHP offers a lot of powerful operators, ranging from arithmetic,
string, and logical operators to operators for assignment, comparison, and
more (see Table 4-1).

Table 4-1. PHP operator types

Operator

Used for

Example

Arithmetic

Basic
mathematics

$a
+ $b

Array

Array union

$a
+ $b

Assignment

Assigning
values

$a
= $b + 23

Bitwise

Manipulating bits within
bytes

12
^ 9

Comparison

Comparing two
values

$a
< $b

Execution

Executing contents of
backticks

`ls -al`

Increment/Decrement

Adding or subtracting
1

$a++

Logical

Boolean
comparisons

$a
and $b

String

Concatenation

$a
. $b

Different types of operators take a different number of
operands:

Unary operators, such as incrementing
($a++) or negation (-$a), take a single operand.

Binary operators, which represent the bulk
of PHP operators (including addition, subtraction, multiplication, and
division), take two operands.

There is one ternary operator, which takes
the form x ? y : z. It’s a terse,
single-line if statement that
chooses between two expressions, depending on the result of a third
one. This conditional operator takes three operands.

Operator Precedence

If all operators had the same precedence, they would be processed
in the order in which they are encountered. In fact, many operators do
have the same precedence—Example 4-5
illustrates one such case.

Example 4-5. Three equivalent expressions

1 + 2 + 3 - 4 + 5
2 - 4 + 5 + 3 + 1
5 + 2 - 4 + 1 + 3

Here you will see that although the numbers (and their preceding
operators) have been moved around, the result of each expression is the
value 7, because the plus and minus
operators have the same precedence. We can try the same thing with
multiplication and division (see Example 4-6).

Example 4-6. Three expressions that are also equivalent

1 * 2 * 3 / 4 * 5
2 / 4 * 5 * 3 * 1
5 * 2 / 4 * 1 * 3

Here the resulting value is always 7.5. But things change when we mix operators
with different precedences in an expression, as in
Example 4-7.

Example 4-7. Three expressions using operators of mixed precedence

1 + 2 * 3 - 4 * 5
2 - 4 * 5 * 3 + 1
5 + 2 - 4 + 1 * 3

If there were no operator precedence, these three expressions
would evaluate to 25, −29, and 12, respectively. But because multiplication
and division take precedence over addition and subtraction, there are
implied parentheses around these parts of the expressions, which would
look like Example 4-8 if
they were visible.

Example 4-8. Three expressions showing implied parentheses

1 + (2 * 3) - (4 * 5)
2 - (4 * 5 * 3) + 1
5 + 2 - 4 + (1 * 3)

Clearly, PHP must evaluate the subexpressions within parentheses
first to derive the semi-completed expressions in Example 4-9.

Example 4-9. After evaluating the subexpressions in parentheses

1 + (6) - (20)
2 - (60) + 1
5 + 2 - 4 + (3)

The final results of these expressions are −13, −57,
and 6, respectively (quite different
from the results of 25, −29, and 12
that we would have seen had there been no operator precedence).

Of course, you can override the default operator precedence by
inserting your own parentheses and force the original results that we
would have seen, had there been no operator precedence (see Example 4-10).

Example 4-10. Forcing left-to-right evaluation

((1 + 2) * 3 - 4) * 5
(2 - 4) * 5 * 3 + 1
(5 + 2 - 4 + 1) * 3

With parentheses correctly inserted, we now see the values
25, −29, and 12, respectively.

Table 4-2 lists
PHP’s operators in order of precedence from high to low.

Table 4-2. The precedence of PHP operators (high to low)

Operator(s)

Type

()

Parentheses

++ −−

Increment/Decrement

!

Logical

* / %

Arithmetic

+ - .

Arithmetic and
string

<< >>

Bitwise

< <= > >=
<>

Comparison

== != === !==

Comparison

&

Bitwise (and
references)

^

Bitwise

|

Bitwise

&&

Logical

||

Logical

? :

Ternary

= += -= *= /= .= %= &= != ^= <<=
>>=

Assignment

and

Logical

xor

Logical

or

Logical

Associativity

We’ve been looking at processing expressions from left to right,
except where operator precedence is in effect. But some operators can
also require processing from right to left. The direction of processing
is called the operator’s associativity.

This associativity becomes important in cases in which you do not
explicitly force precedence. Table 4-3 lists all the
operators that have right-to-left associativity.

Table 4-3. Operators with right-to-left associativity

Operator

Description

NEW

Create a new
object

!

Logical
NOT

~

Bitwise
NOT

++ −−

Increment and
decrement

+ −

Unary plus and
negation

(int)

Cast to an
integer

(double)

Cast to a
float

(string)

Cast to a
string

(array)

Cast to an
array

(object)

Cast to an
object

@

Inhibit error
reporting

=

Assignment

For example, let’s take a look at the assignment operator in Example 4-11, where three variables are
all set to the value 0.

Example 4-11. A multiple-assignment statement

<?php
$level = $score = $time = 0;
?>

This multiple assignment is possible only if the rightmost part of
the expression is evaluated first and then processing continues in a
right-to-left direction.

Note

As a PHP beginner, you should learn to avoid the potential
pitfalls of operator associativity by always nesting your
subexpressions within parentheses to force the order of evaluation.
This will also help other programmers who may have to maintain your
code to understand what is happening.

Relational Operators

Relational operators test two operands and
return a Boolean result of either TRUE or FALSE. There are three types of relational
operators: equality,
comparison, and logical
operators.

Equality operators

The equality operator, which we’ve already encountered a few
times in this chapter, is == (two
equals signs). It is important not to confuse it with the = (single equals sign) assignment operator.
In Example 4-12, the
first statement assigns a value and the second tests it for
equality.

As you see, returning either TRUE or FALSE, the equality operator enables you to
test for conditions using, for example, an if statement. But that’s not the whole
story, because PHP is a loosely typed language. If the two operands of
an equality expression are of different types, PHP will convert them
to whatever type makes best sense to it.

For example, any strings composed entirely of numbers will be
converted to numbers whenever compared with a number. In Example 4-13, $a and $b
are two different strings and we would therefore expect neither of the
if statements to output a
result.

However, if you run the example, you will see that it outputs
the number 1, which means that the first if statement evaluated to TRUE. This is because both strings were
first converted to numbers, and 1000 is the same numerical value as
+1000.

In contrast, the second if
statement uses the identity operator—three equals
signs in a row—which prevents PHP from automatically converting types.
$a and $b are therefore compared as strings and are
now found to be different, so nothing is output.

As with forcing operator precedence, whenever you feel there may
be doubt about how PHP will convert operand types, you can use the
identity operator to turn off this behavior.

In the same way that you can use the equality operator to test
for operands being equal, you can test for them
not being equal using !=, the inequality
operator. Take a look at Example 4-14, which is a
rewrite of Example 4-13 in
which the equality and identity operators have been replaced with
their inverses.

As you might expect, the first if statement does not output the number 1,
because the code is asking whether $a and $b
are not equal to each other numerically.

Instead, it outputs the number 2, because the second if statement is asking whether $a and $b
are not identical to each other in their present
operand types, and the answer is TRUE; they are not the same.

Comparison operators

Using comparison operators, you can test for more than just
equality and inequality. PHP also gives you > (is greater than), < (is less than), >= (is greater than or equal to), and
<= (is less than or equal to) to
play with. Example 4-15 shows
these operators in use.

Example 4-15. The four comparison operators

<?php
$a = 2; $b = 3;
if ($a > $b) echo "$a is greater than $b<br />";
if ($a < $b) echo "$a is less than $b<br />";
if ($a >= $b) echo "$a is greater than or equal to $b<br />";
if ($a <= $b) echo "$a is less than or equal to $b<br />";
?>

In this example, where $a is
2 and $b is 3,
the following is output:

2 is less than 3
2 is less than or equal to 3

Try this example yourself, altering the values of $a and $b, to see the results. Try setting them to
the same value and see what happens.

Logical operators

Logical operators produce true-or-false results, and therefore
are also known as Boolean operators. There are four of them (see Table 4-4).

Table 4-4. The logical operators

Logical
operator

Description

AND

TRUE if both operands are TRUE

OR

TRUE if either operand is TRUE

XOR

TRUE if one of the two operands is
TRUE

NOT

TRUE if the operand is FALSE or FALSE if the operand is TRUE

You can see these operators used in Example 4-16. Note that the ! symbol is required by PHP in place of the
word NOT. Furthermore, the
operators can be lower- or uppercase.

This example outputs NULL,
1, 1, NULL,
meaning that only the second and third echo statements evaluate as TRUE. (Remember that NULL—or nothing—represents a value of
FALSE.) This is because the
AND statement requires both
operands to be TRUE if it is going
to return a value of TRUE, while
the fourth statement performs a NOT
on the value of $a, turning it from
TRUE (a value of 1) to FALSE. If you wish to experiment with this,
try out the code, giving $a and
$b varying values of 1 and 0.

Note

When coding, remember to bear in mind that AND and OR have lower precedence than the other
versions of the operators, && and ||. In complex expressions, it may be
safer to use && and
|| for this reason.

The OR operator can cause
unintentional problems in if
statements, because the second operand will not be evaluated if the
first is evaluated as TRUE. In
Example 4-17, the function
getnext will never be called if
$finished has a value of 1.

Example 4-17. A statement using the OR operator

<?php
if ($finished == 1 OR getnext() == 1) exit;
?>

If you need getnext to be
called at each if statement, you
could rewrite the code as has been done in Example 4-18.